The HST was probably an amazing train when it was new, but it is old and the IET is a much better train. Yes, the seats could be a little better but having doors that are easy to open from the inside and being electric are huge advantages. If you want the train to be an attractive way of getting from A to B, a good start is to not have the major stations full of diesel exhaust.
I agree, the HST was an amazing train when it was new. Compared with the typical train of the time made up of Mark 2 coaches of various vintages, sometimes with a sprinkling of Mark 1 coaches, the HST was quieter, the brakes were silent in contrast to cast iron clasp brakes, all the train was air conditioned and the bogies had secondary air suspension which removed most of the high frequency vertical 'nervousness' from the ride which was a feature of the B4 bogies under the coaches it replaced. The much discussed smell from hot brake pads was largely resolved within a couple of years by adding flaps to the air conditioning air intakes which operated when the brakes were applied. Observed externally disc braked wheels made very much less noise when running past at speed than did clasp braked wheels — this was especially noticeable on the west coast main line when mixed formations of Mark 2 and Mark 3 stock went past.
With the HST BR increased train top speeds by 25% — overnight. The difference between older stock and the HST was as if one had entered a different world.
My take on the IEP/Class 80x/whatever is that it is an enormous missed opportunity and has several design flaws. BR spent a lot of time and effort in improving the 'mechanics' of the trains. After the BT10 bogie of the Mark 3 it introduced the Series 3 bogie which did away with swing links in the secondary suspension and used the flexibility of the air bags to simplify the secondary suspension and improve the ride. The Series 4 bogie was a further development and was in my experience the best riding of all BR's designs, at least up to 90mph the top speed of the only class of trains, the 158, to use it.
I would have expected that 50 years after the inception of the BT10 bogie the IEP would have offered another step change in ride quality and offer a silky smooth, quiet, vibration free ride. It doesn't. The bogie suspension is not well controlled, I hear and feel bangs, sometimes quite loud, when running over some switch and crossing work and sometimes, inexcusably, on straight track as well. There is a residual highish frequency vibration through the floor and seats and one train I was travelling on started a high amplitude vibration at speeds in excess of 115mph. It felt like an out-of-balance wheel in a car and reminded me of a slightly run-down B4 bogie under a Mark 1 coach. I have never travelled on a well maintained Mark 3 coach - the design of which commenced in 1969 - which exhibited such behaviour.
It is almost impossible to keep the windows on an IEP clean, the glass is more recessed in the frames than is the case with almost any recent rolling stock operated in the UK and the vortices generated off the edge deposit grime on the glass. The deep door pockets are as bad - why produce a design which requires more maintenance to keep it looking clean than needed?
I for one am very disappointed with the IEP. It is, at best, only marginally better than the 40 year old coaches it is replacing each of which has several million miles on the clock. I would have expected something much, much better — smoother, quieter, more sophisticated and internally elegant.
Edit: Corrected typos.